6 History of Luminescence 



cence is connected with the element phosphorus. Many organic 

 compounds are chemiluminescent when oxidized in aqueous solu- 

 tion. Living organisms have manufactured some such chemilumi- 

 nescent substance which is utilized in 



BiOLUMiNESCENCE, the chemiluminesccnt emission of light by 

 living animals and plants. 



Many luminescent phenomena involve a combination of the 

 above types. For example, if chemiluminescent substances are dis- 

 solved in a liquid through which intense sound waves are passing, 

 the formation of active oxygen may result in a sonic-chemilumi- 



NESCENCE. 



Since the formulation of the quantum theory by Max Planck in 

 1900, and the later developments of quantum mechanics, together 

 with the modern conception of the atom as a highly complex entity, 

 so many new terms have been introduced into the subject of lumi- 

 nescence that a physicist of the eighteen-nineties would be quite 

 unable to comprehend the phraseology or follow the theory. He 

 would be immediately involved with photons, excitation potentials, 

 ionization potentials, secondary electrons, electron shells, permitted 

 transitions, rotational quantum numbers, the lifetime of an excited 

 state, metastable states, and the cross-section of quenching. Even 

 an electron-volt and a molecular lattice would present novelties 

 requiring precise definition. This " History of Luminescence " does 

 not pretend to cover the newer field. For all practical purposes, it 

 stops with the discovery of the electron. 



However, the essentials of the present view regarding mechanism 

 of light emission can be obtained from the following brief and 

 simplified statement. In every type of luminescence, modern theory 

 postulates that some atom or molecule acquires excess energy 

 (spoken of as an " excited " atom or molecule) , with displacement 

 of an electron to a higher energy level. The energy for this dis- 

 placement may come from exciting radiation (light. X-rays, various 

 particles) or from electrical energy (electrons) or mechanical energy 

 or chemical energy. In this way the various types of luminescence 

 are excited. When the electron returns to the lower level, a quantum 

 of light is emitted, the sum total of these quanta constituting the 

 luminescence. As contrasted with the very broad spectra of incandes- 

 cence or temperature radiation, the spectra of luminescences are 

 narrow bands or lines in one or another spectral region, thus giving 

 a definite color to the light. 



The emission is quite different from the expected black body radiation for the tem- 

 perature and later became known as a candoluminescence, but the word is rarely 

 used at the present time. 



